Good Design More on (ab)use of PIC protection diodes & bad design
Jim Hartmann email (remove spam text)
The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss :
-0.6V to VDD+0.6V".
I think there is some confusion as to what the DC Characteristics "Input
Low Voltage" and "Input High Voltage" mean. I believe that these
specifications do not dictate limits to what can be applied to the pins.
These specifications dictate what Microchip guarantees will be the
thresholds of the input logic circuit. An input voltage will produce a
zero input guaranteed if the input voltage is VSS. But the threshold could
be as high as 0.8V maximum (ttl buffer).
Anyone care to differ?
Alan Pearce <RL.AC.UK>MITVMA.MIT.EDU> on 10/11/1999 10:38:22 AM A.B.Pearce
Please respond to pic microcontroller discussion list
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Subject: Re: PICs: Good Design More on (ab)use of PIC protection diodes &
>Would you kindly point me to a page and line where it says that a negative
>voltage WRT Vss on a pin violates the operating conditions?
I do not have the data sheet for the 16C622 which I believe you are using,
looked up the sheet for the 16C6x (DS30234D) and page 183 lists "Maximum
any pin to Vss as -0.3V to Vdd+0.3V, i.e less than a diode voltage drop
the supply rails. This is listed the same through subsequent pages for the
various models of this PIC.
Page 186 gives "Standard Operating Conditions" i.e. the conditions it would
expect to operate under as Parameter D030 - Input Low Voltage Min Vss - not
The next parameter is "Input High Voltage" Max Vdd - not Vdd+0.3V. the
pages list the same for the other models of this family.
I also checked the data sheet I have for 16F87x (DS30292A) and this gives
same set of voltages for the same parameters. I repeat that "Maximum
are the limits that will not destroy the device, but are not "standard
conditions". All micros, digital and analogue ICs, transistors diodes and
practically any component I can think of is specified this way.
An application note which shows a resistor being used off the mains to
pin does not necessarily give licence to use a chip like that. A collegue I
worked with always said "It is not what is in the circuit on the
note, but what they left out that is important", and he is right. The
used in the app note may have happened to work because an adjacent pin
used, or the chip was a preproduction one that had a different mask
the circuit designer was lucky and got away with it. It may even be that
circuit he used had the same problem you do, but the circuit that got
did not show the mods used to get around the problem.
See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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